/Engine/Source/Developer/SourceControl/Public/ISourceControlState.h
Get the revision that we should use as a base when performing a three way merge, does not refresh source control state
a revision identifier or NULL if none exist
virtual [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< class [ISourceControlRevision](API\Developer\SourceControl\ISourceControlRevision), ESPMode::ThreadSafe > GetBaseRevForMerge() const